A Study of the Influence of Lithium Salt Anions on Oxygen Reduction Reactions in Li-Air Batteries

The influence of lithium salts on O 2 reduction reactions (ORR) in 1, 2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME) has been investigated. Microelectrode studies in a series of tetrabutylammonium salt (TBA salt)/DME-based electrolytes showed that O 2 solubility and diffusion coefficient are not significantly affected by the electrolyte anion. The ORR voltammograms on microelectrodes in these electrolytes exhibited steady-state limiting current behavior. In contrast, peak-shaped voltammograms were observed in Li + -conducting electrolytes suggesting a reduction of the effective electrode area by passivating ORR products as well as migration-diffusion control of the reactants at the microelectrode. FT-IR spectra have revealed that Li + ions are solvated to form solvent separated ion pairs of the type Li + (DME) n PF 6 − and Li + (TEGDME)PF 6 − in LiPF 6 -based electrolytes. On the other hand, the contact ion pairs (DME) m Li + (CF 3 SO 3 − ) and(TEGDME)Li + (CF 3 SO 3 − ) appear to form in LiSO 3 CF 3 -containing electrolytes. In the LiSO 3 CF 3 –based electrolytes the initial ORR product, superoxide (O 2 − ), is stabilized in solution by forming [(DME) m-1 (O 2 − )]Li + (CF 3 SO 3 − ) and [(TEGDME)(O 2 − )]Li + (CF 3 SO 3 − ) complexes. These soluble superoxide complexes are able to diffuse away from the electrode surface reaction sites to the bulk electrolyte in the electrode pores where they decompose to form Li 2 O 2 . This explains the higher capacity obtained in Li/O 2 cells utilizing LiCF 3 SO 3 /TEGDME electrolytes.